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Acta Physica Sinica

Quantum key distribution based on time coding and phase coding

Jing Zhang, Wang Fa-Qiang, Zhao Feng, Lu Yi-Qun, Songhao Liu, 华南师范大学信息光电子科技学院，光子信息技术实验室，广州 510006

Published Jan 1, 2008• 1 citation
Publisher

Abstract

A new quantum key distribution scheme is proposed to realize both phase coding and time coding based on two unbalanced Mach-Zehnder interferometers. The pulses which are discarded in the phase coding scheme can be coded in time coding, so the useful bit rate in the present scheme can be doubled. At the same time, the phase coding keys and the time coding keys are obtained. We can use one group of keys to communicate and the other one to guard against eavesdropping, or combine both of them to form new keys. This scheme has favorable application prospect.

DOI
10.7498/aps.57.4941

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A new quantum key distribution scheme is proposed to realize both phase coding and time coding based on two unbalanced Mach-Zehnder interferometers. The pulses which are discarded in the phase coding scheme can be coded in time coding, so the useful bit rate in the present scheme can be doubled. At the same time, the phase coding keys and the time coding keys are obtained. We can use one group of keys to communicate and the other one to guard against eavesdropping, or combine both of them to form new keys. This scheme has favorable application prospect.
We propose one kind of improved QKD scheme based on time coding and phase coding. The pulse which had been discarded on the BB84 protocol phase coding QKD system is used to realize the time coding protocol, thus the useful bits rate in the present scheme can be doubled. At the same time, the phase coding keys and the time coding keys are obtained, we may combine both as new keys which can enhancethe generated rate and the sensitivity of monitor. At Bob’s side, a Fraday-Michelson is used instead of a traditional M-Z interferometer, which improves the stability of the interference visibility. With the proposed experimental setup, a stable quantum key distribution was performed over 90 km fiber. It is shown in our experiment that the characteristics of our system are as follows: secure, stable and economical.
In this study, we demonstrate the possibility of realizing the quantum key distribution system by temporal coding of time-bin qubits that are prepared and measured using unbalanced Mach-Zehnder interferometers. We also show that, to exclude a clear a priori identification of time-bin qubits with equal bases, their encoding with the symbols 0 and 1 can be accomplished by delaying one of them by half the clock interval. The uniqueness of measurements of qutrits in this algorithm is shown to be based on the ability of quantum particles in β-states to interfere with the probability amplitudes at the ports of the system interferometers, as well as the absence of collisions of signaling states of the qutrits in adjacent clock intervals. We describe a method for controlling such collisions, including those that are based on qubits prepared from quasi-one-photon coherent laser states.
A cryptographic analysis is presented of a new quantum key distribution protocol using phase-time coding. An upper bound is obtained for the error rate that guarantees secure key distribution. When no counts are detected in the control time slot, the protocol guarantees secure key distribution if the bit error rate in the sifted key does not exceed 50%. This protocol partially discriminates between errors due to system defects (e.g., imbalance of a fiber-optic interferometer) and eavesdropping. In the absence of eavesdropping, the counts detected in the control time slot are not caused by interferometer imbalance, which reduces the requirements for interferometer stability.
A cryptographic analysis is presented of a new quantum key distribution protocol using phase-time coding. An upper bound is obtained for the error rate that guarantees secure key distribution. It is shown that the maximum tolerable error rate for this protocol depends on the counting rate in the control time slot. When no counts are detected in the control time slot, the protocol guarantees secure key distribution if the bit error rate in the sifted key does not exceed 50%. This protocol partially discriminates between errors due to system defects (e.g., imbalance of a fiber-optic interferometer) and eavesdropping. In the absence of eavesdropping, the counts detected in the control time slot are not caused by interferometer imbalance, which reduces the requirements for interferometer stability.
Security proof of practical quantum key distribution (QKD) has attracted a lot of attentions in recent years. Most of real-life QKD implementations are based on phase-coding BB84 protocol, which usually use Unbalanced Mach-Zehnder Interferometer (UMZI) as the information encoder and decoder. However, the long arm and short arm of UMZI will introduce different loss in practical experimental realizations, the state emitted by Alice's side is nolonger perfect BB84 states correspondingly. In this paper, we will give the security analysis in this situation. Counterintuitively, active compensation for this different loss will only lower the secret key bit rate.
We present a parallel quantum key distribution scheme, where a Mach–Zehnder (M–Z) interferometer is applied to achieve a subcarrier multiplexing quantum key distribution. The polarization of each sideband of the composite signal and their controlling factors are analyzed. The factors, such as intermodulation interference and laser source phase noise, which might increase quantum bit error, are studied. The simulation of the quantum bit error rate of the four different sidebands shows that this type of modulation can meet the bit error rate requirement of quantum key distribution.